A "color" digital image processing system typically includes a digitizer, an image processing system, and an output display device. The digitizer samples a color image and produces three separate color digital images. The colors are usually red, green and blue. Other colors can be used. Also, different visible regions of the electromagnetic spectrum can be used such as IR and UV. When the color digital images are combined, they should correspond to the original color image. The image processing system operates upon the color digital images using a variety of well known image enhancing techniques. The enhanced color digital images then are applied on a pixel-by-pixel basis to an output display device. Examples of output display devices are printers such as laser output printers and CRT systems. In CRT systems which display color images, the digital pixels of each color digital image are converted to analog signals (e.g. red, green and blue), each of which controls its own electron beam gun. Each gun has the usual function of producing a beam of electrons. Each beam excites a different color phosphor in the screen of a CRT. With all three guns operating, the screen of the CRT produces red, green and blue colors and their color mixtures.
Frequently, the color digital image pixels have many more discrete brightness levels than the corresponding brightness levels of the output display device. If the pixels of a color digital image are transformed into the corresponding output levels of the display device, the resulting output is generally of unsatisfactory quality. A number of techniques have been used with varying degrees of success, to modify the brightness levels of the color digital images to make them compatible with the available brightness levels of an output device. For example, U.S. Pat. No. 3,979,555 issued Sept. 7, 1976 to Opittek et al discloses a histogram equalization method. The equalized histogram of a digital image is substantially flat across its entire range. In other words, each brightness level contains about the same number of pixels as every other brightness level. Histogram equalization methods are quite effective for monochrome images. When they are used to transform the brightness levels of color digital images, they have not produced satisfactory quality color output images.